U.S. patent number 4,087,800 [Application Number 05/736,792] was granted by the patent office on 1978-05-02 for conveyor belt monitoring system.
This patent grant is currently assigned to The B. F. Goodrich Company. Invention is credited to Maw-Huei Lee.
United States Patent |
4,087,800 |
Lee |
May 2, 1978 |
Conveyor belt monitoring system
Abstract
A monitoring system for detecting a longitudinal rip in a
conveyor belt senses the presence of the rip and simultaneously
produces a warning signal and stops the operation of the conveyor
system. The system includes a series of sensor circuits embedded in
the conveyor belt in spaced relationship along the length of the
belt and a series of alarm circuits positioned at intervals along
the upper run of the conveyor belt but out of actual contact with
the conveyor belt. Each sensor unit embedded in the conveyor belt
consists of a loop of conductive wire or ribbon positioned
transversely across substantially the width of the conveyor belt
and a coil wound on a powder core with its terminals in electrical
connection with the ends of the loop of conductive wire or ribbon.
When the conveyor belt is free of longitudinal rips, the sensor
circuit as it passes in close proximity to the alarm circuit has no
noticeable effect on the alarm circuit. However, when a
longitudinal rip develops in the conveyor belt and parts the loop
of wire in one of the sensor circuits thereby breaking the
electrical circuit of that sensor circuit, the effected sensor
circuit as it passes in close proximity to an alarm circuit causes
the triggering of the alarm circuit that sets off a warning alarm
and stops the operation of the conveyor system.
Inventors: |
Lee; Maw-Huei (Brecksville,
OH) |
Assignee: |
The B. F. Goodrich Company
(Akron, OH)
|
Family
ID: |
24961316 |
Appl.
No.: |
05/736,792 |
Filed: |
October 29, 1976 |
Current U.S.
Class: |
340/676;
198/810.02; 331/65 |
Current CPC
Class: |
B65G
43/02 (20130101) |
Current International
Class: |
B65G
43/02 (20060101); G08B 021/00 () |
Field of
Search: |
;340/259,258C,58
;200/61.22 ;198/502,505 ;331/64,65 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell, Sr.; John W.
Assistant Examiner: Nowicki; Joseph E.
Claims
I claim:
1. A monitoring system for detecting the presence of longitudinal
rips in a conveyor belt, said monitoring system comprising an
endless flexible conveyor belt, a series of sensor circuits
embedded in said conveyor belt and spaced relative to each other at
intervals along the length of said conveyor belt, each said sensor
circuit consisting of an open loop of conductive wire that has
terminal ends and that extends transversely across substantially
the width of said conveyor belt and an inductor coil means that has
terminal ends and that is wound on a powder core, the terminal ends
of said inductor coil means in each said sensor circuit being in
electrial connection with the terminal ends of said loop of
conductive wire of the sensor circuit, at least one alarm circuit
positioned adjacent to the path of advance of said conveyor belt,
said alarm circuit including in electrical connection an inductor
coil means wound on a powder core, capacitance means, resistor
means, oscillator means for generating an alternating signal of
constant frequency and a triggering circuit responsive to a change
in the voltage across said inductor coil means of the alarm
circuit, said inductor coil means of said alarm circuit being
positioned in close proximity to said conveyor belt at a location
so that the inductor coil means of each said sensor circuit will
pass in close proximity to the inductor coil means of said alarm
circuit as the said conveyor belt is advanced, said inductor coil
means of said alarm circuit and the said capacitance means in
electrical connection therewith being in a "primary circuit" that
is in electrical resonance at the frequency of the signal generated
by said oscillator means in said alarm circuit, said inductor coil
means of each said sensor circuit containing a number of turns of
wire in the coil to cause the said inductor coil means to be in
electrical resonance with its distributed capacitance at the
frequency of the said signal generated by said oscillator means
within the said alarm circuit with the circuit of the sensor
circuit open and during the time the said inductor coil means of
the open sensor circuit passes in close proximity to said inductor
coil means of said alarm circuit.
2. A monitoring system according to claim 1 wherein said system
includes a series of said alarm circuits spaced at intervals along
the path of advance of said conveyor belt and wherein said
oscillator means of each said alarm circuit is structured to emit a
signal of essentially the same frequency during the operation of
the said monitoring system.
3. A monitoring system according to claim 2 wherein said inductor
coil means of said sensor circuits and said inductor coil means of
said alarm circuits are wound on cores comprised of powdered
ferromagnetic material.
4. A monitoring system according to claim 2 wherein said inductor
coil means of said sensor circuits and said inductor coil means of
said alarm circuits are wound on cores comprised of powdered
ferrimagnetic material.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a monitoring system for detecting
a longitudinal (lengthwise) rip in a conveyor belt before the rip
reaches a size sufficient to cause excessive or irreparable damage
to the conveyor belt or to the conveyor drive system. In large
endless conveyor belt systems that are used for transporting bulk
materials such as coal, mineral rock and ores over long distances,
it is not uncommon for sharp or jagged pieces of the conveyed
material as they are dropped onto the conveyor belt to penetrate
into the conveyor belt. With continued use of the conveyor belt,
the damaged area often can develop into a rip that progressively
increases in length longitudinally along the belt until the rip has
grown in size sufficient to require the replacement of a large
segment of the belt or, sometimes, the replacement of the entire
belt. Additionally, if the rip becomes too extensive, the conveyor
belt itself may pull apart and become jammed in the drive mechanism
of the conveyor system. Accordingly, it has long been recognized
that a monitoring system for early detection of longitudinal rips
in conveyor belts is desirable.
A number of such monitoring systems have been proposed, but
generally the systems are expensive and quite complex in their
circuitry thereby increasing the number of potential sources for
component failure. Examples of three monitoring systems which
previously have been proposed are described in U.S. Pat. Nos.
3,651,506; 3,656,137 and 3,792,459.
The monitoring system described in U.S. Pat. No. 3,651,506 utilizes
closed loops of conductive wire embedded in the belt at spaced
intervals along the length of the conveyor belt. At selected
locations along the path of advance of the belt are positioned a
frequency transmitter and a receiver, the transmitter being
positioned at one side of the belt with the receiver positioned at
the other side of the belt opposite the transmitter. As long as the
loops of wire embedded in the belt are not broken, a signal emitted
by the transmitter is electromagnetically transmitted by the closed
wire loop to the receiver which emits an output signal to an
analyzing circuit. The regularly timed pulsing signals from the
receiver to the analyzing circuit prevents the triggering of the
alarm circuit and the shut-down of the conveyor system. However, if
a longitudinal rip develops in the conveyor belt and one of the
wire loops becomes broken, no output signal is emitted from the
receiver as the broken loop passes between the transmitter and
receiver. The interruption to the regular pattern of pulsing
signals from the receiver to the analyzing circuit causes the
triggering of the alarm circuit and the shut-down of the belt drive
mechanism.
U.S. Pat. No. 3,792,459 describes a monitoring system which
includes a number of single wire conductors embedded in the belt at
spaced intervals along the length of the belt. The conductors
extend transversely across substantially the entire width of the
belt. As the belt advances the conductors pass over a signal
transmitter plate positioned beneath one edge of the belt and a
detector plate positioned beneath the opposite edge of the belt.
When an unbroken conductor passes over the signal transmitter plate
an electrical signal is capacitively induced in the conductor which
caues a signal to be capacitively induced at the detector plate.
However, when the conductor is broken (for example, by the
occurrence of a longitudinal rip in the belt), there will not be a
signal inductively produced at the detector plate and an
interruption to the regular pulsating pattern of signals emitted by
the detector plate will occur causing the triggering of the alarm
circuitry and a shut-down of the system.
In U.S. Pat. No. 3,656,137, a monitoring system is described that
utilizes a series of spaced conductive wire loops which extend
transversely across substantially the entire width of th conveyor
belt. Each wire loop is in electrical connection with a turned
circuit consisting of a capacitor in parallel electrical connection
with an inductance coil. The wire loop serves to "short circuit"
the tuned circuit with which it is associated as long as the wire
loop remains unbroken. A stationary tuned circuit monitor unit is
positioned at the side of the conveyor belt. As the belt is
advanced, the wire loops embedded in the belt are caused to pass by
the stationary tuned circuit monitor unit. As long as the wire
loops remain unbroken, their passage by the monitor unit causes an
insufficient effect on the monitoring unit to trigger the alarm
circuitry. However, if a rip develops in the belt and breaks a wire
loop, the tuned circuit formed by the inductance coil and capacitor
with which the wire loop is associated no longer is "short
circuited" and as it passes the monitor unit electromagnetically
couples with the tuned circuit of the monitor unit resulting in a
reduction in the amplitude of the output of the monitor unit which,
in turn, triggers the alarm circuit. Although the monitoring system
described in U.S. Pat. No. 3,656,137 is not subject to many of the
deficiencies inherent in other systems preciously proposed, the use
of a capacitor in the wire loop circuits embedded in the belt
creates a source of possible malfunction in the system since the
capacitors are likely to be damaged and rendered non-functional by
the dumping of bulk material onto the belt.
SUMMARY OF THE INVENTION
The present invention provides a reliable monitoring system for
detecting a longitudinal rip in a conveyor belt before it has
progressed in size sufficiently to require the replacement of an
unduly large segment of the belt. The system includes a series of
sensor circuits embedded in the conveyor belt at spaced intervals
along the length of the belt and a series of alarm circuits
positioned at intervals along the length of the conveyor belt in
close proximity to but spaced from the conveyor belt. Each of the
sensor circuits embedded in the conveyor belt consists of a loop of
conductive wire (either of round or flattened cross-section)
extending transversely across substantially the width of the
conveyor belt and an inductor coil wound on a powder core with the
terminals of the inductor coil in electrical connection with the
ends of the loop of conductive wire. The alarm circuit includes an
inductor coil wound on a powder core and a capacitor that are in a
circuit that is tuned to electrical resonance at the frequency of a
signal generated within the alarm circuit. The number of turns of
wire in the inductor coil of each sensor circuit are chosen so that
the inductor coil of the sensor cicuit is in electrical resonance
with the distributed capacitance of the inductor coil at the
frequency of the signal generated within the alarm circuit whenever
the loop of wire in the sensor circuit is broken. As the conveyor
belt is advanced the inductor coil of the sensor circuit passes in
close proximity to the inductor coil of the alarm circuit. As long
as the loop of wire of the sensor circuit remains unbroken, the
passage of the inductor coil of the sensor circuit in close
proximity to the inductor coil of the alarm circuit has
insufficient effect on the voltage amplitude across the inductor
coil of the alarm circuit to cause the triggering of the alarm
circuit. However, if a longitudinal rip in the conveyor belt
develops and becomes of sufficient length to cause the breaking of
the loop of wire of one of the sensor circuits embedded in the
conveyor belt, the passage of the inductor coil of the affected
sensor circuit in close proximity to the inductor coil of the alarm
circuit will cause a sufficient drop in the amplitude of the
voltage across the inductor coil of the alarm circuit to trigger
the alarm circuit. The triggering of the alarm circuit sets off a
warning signal or stops the operation of the conveyor system or
both. The warning signal may be a visual signal or an audible
signal or may consist of both audible and visual signals.
The invention will be more fully understood by reference to the
following description of a preferred embodiment of the invention
when read in connection with the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic plan view, partly broken away, of a portion
of a conveyor belt traveling over support rollers and illustrates a
series of sensor circuits embedded at spaced intervals along the
conveyor belt and the inductor coils of two alarm circuits
positioned at intervals along the path of advance of the conveyor
belt and located so that the inductor coils of the sensor circuits
will pass in close proximity to the inductor coils of the sensor
circuits;
FIG. 2 is a section view of the conveyor installation of FIG. 1
taken along line 2--2 of FIG. 1;
FIG. 3 is a block diagram illustrating the preferred embodiment of
this invention;
FIG. 4 shows a circuit diagram illustrating a triggering circuit
which may be used with the embodiment of this invention shown in
FIG. 3;
FIG. 5 is a schematic plan view, partly broken away, illustrating
the sensor circuit employed in the preferred embodiment of this
invention; and
FIG. 6 illustrates the change in voltage amplitude across the
inductor coil of the alarm circuit of the embodiment of this
invention shown in FIG. 1 during the period that the inductor coil
of a sensor circuit in which the loop wire has been broken passes
in close proximity to the inductor coil of the alarm circuit as the
conveyor belt is advanced.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings, the conveyor system 10 includes an
endless non-metallic flexible conveyor belt 11 disposed around
conventional drive and idler rolls (not shown) and supported by
rollers 12,12 mounted for rotation in supports 13,13 and positioned
at intervals along the length of the conveyor system.
In accordance with this invention, a series of sensor circuits
14,14 are embedded in the conveyor belt 11 at spaced intervals
along the length of the belt 11. The interval between adjacent
sensor circuits 14,14 can vary, a distance between adjacent sensor
circuits 14,14 ranging from 5 to 40 feet being a desirable spacing
with a spacing between adjacent sensor circuits 14,14 of about 20
feet being preferred. The spacing between adjacent sensor circuits
14,14 need not be uniform along the length of the conveyor belt
since the monitoring system of this invention does not depend upon
the interruption of a pulsing signal at uniform intervals for
triggering the alarm circuit.
Each sensor circuit 14 consists of an open loop of conductive wire
15 that extends transversely across substantially the width of the
conveyor belt 11 and an inductor coil 16 wound on a powder core 17
with the terminals of inductor coil 16 in electrical connection
with the ends of the loop of conductive wire 15, as illustrated in
FIG. 5. The inductor coil 16 and powder core 17 of the sensor
circuit may be enclosed in a housing 18 to protect the inductor
coil 16 and powder core 17 from damage. However, housing 18, if
used, is not considered to constitute a component of the sensor
circuit. Preferably, the sensor circuits 14,14 are positioned
between the belt carcass and the cover on the inner periphery of
the conveyor belt to remove them from being closely adjacent to the
surface of the belt onto which the bulk material to be conveyed is
charged.
The gauge of wire from which the loop of conductive wire 15 of the
sensor circuit 14 is formed should be sufficiently smll so that a
longitudinal rip in the conveyor belt 11 that extends through a
sensor circuit 14 (such as the rip 19 depicted in FIG. 1) will
cause the breakage of the loop of wire 15. The gauge of wire from
which the loop of wire 15 of the sensor circuit is formed should
not be so small, however, that the loop of wire 15 will be broken
merely as a result of the conveyor belt 11 being fixed as it is
advanced around the drive and idler rolls of the conveyor system.
Desirably, a strand of wire is used. A No. 22 gauge strand of
copper wire has been found to be satisfactory for use in forming
the loops of wire 15,15 of the sensor circuits 14,14.
As shown in FIG. 3, the alarm circuits 20,20 of the monitoring
system each includes an inductor coil 21 wound on a powder core 22
and serially connected with resistor 23 and a generating source of
constant frequency alternating current such as oscillator 24. A
condenser 25 is connected across the terminals of inductor coil 21
and an alarm triggering circuit 26 (which may consist of the
circuitry shown in FIG. 4 and which will be described below in
greater detail) is connected across the terminals of resistor 23.
The inductor coils 21,21 of the alarm circuits 20,20 are positioned
adjacent to the edge of conveyor belt 11 and are located so that as
the conveyor belt 11 is advanced the inductor coil 16 of each
sensor circuit 14 passes sequentially in close proximity to the
inductor coil 21 of each alarm circuit 20. The inductor coils 21,21
may be retained in fixed position by securing the inductor coil 21
to a bracket 27 which, in turn, is secured to an appropriately
located support 13. Desirably, inductor coil 16 and inductor coil
21 will pass within two to three inches of each other as the sensor
circuit 14 is advanced pass the alarm circuit 20.
Cores 17 and 22 around which inductor coils 16 and 21 respectively
are wound may be made either of powdered ferromagnetic material or
powdered ferrimagnetic material held together with a binder. An
example of a powdered ferromagnetic material is pure carbonyl iron
powder. The powdered ferrimagnetic materials from which powdered
cores usually are made include powdered ferrites of the spinel,
magnetoplumbite, or garnet types.
The circuit of each alarm circuit 20,20 that includes inductor coil
21, resistor 23 and condenser 25, the "primary circuit," is
designed to be tuned to electrical resonance at the frequency of
the signal generated by oscillator 24. The condition of resonance
can be achieved by a proper selection of the inductor coil 21 and
condenser 25. If a particular condenser 25 is selected for use in
the "primary circuit," an inductor coil 21 will need to be selected
that has the correct number of turns of wire in the coil that will
produce the desired resonant condition at the frequency of the
signal emitted by oscillator 24. The correct number of turns of
wire which should be used in the conductor coil 21 can be
determined by varying the number of turns of wire in the coil until
the voltage across the coil is at a maximum (indicating that the
circuit is in electrical resonance). The alarm circuits 20,20
should be designed so that the inductor coil 21 of each alarm
circuit have the same construction.
Also, the circuit of each sensor circuit 14,14 that includes the
inductor coil 16 and the wire loop 15, the "secondary circuit," is
designed so that it exhibits electrical resonance at the frequency
of the signal generated by oscillator 24 when the loop of wire 15
is broken (creating an "open circuit"). The desired condition of
electrical resonance in the "secondary circuit" is achieved by the
proper selection of the ratio of the number of turns of wire in
inductor coil 16 to the number of turns of wire in inductor coil
21. In order to determine the number of turns of wire to be used in
the inductor coil 16 to produce the desired condition of resonance
in the sensor circuit, the number of turns of wire in the inductor
coil 16 is varied and the voltage across inductor coil 16 measured
for the varying number of turns of wire as inductor coil 16 is
moved in close proximity to the inductor coil 21 of an alarm
circuit 20 with the "secondary circuit" open (which condition is
obtained by disconnecting one connection of the wire loop 15 with
coil 16) until the number of turns of wire in inductor coil 16 that
produces the greatest voltage across inductor coil 16 is
determined. The number of turns of wire in coil 16 that produces
the greatest voltage across coil 16 is the number of turns of wire
that produces electrical resonance in the "secondary circuit" of
the sensor circuit 14 at the frequency of the signal emitted by
oscillator 24. Alternatively, with the "primary circuit" of an
alarm circuit 20 tuned to resonance and the "secondary circuit" of
the sensor circuit being tuned to resonance "open" the number of
turns of wire in inductor coil 16 are varied and the voltage across
condenser 25 of the alarm cicuit 20 measured (for each of the
varying number of turns of wire in the coil 16) as the coil 16
passes in close proximity to coil 21 of the alarm circuit 20 until
the voltage across the condenser 25 of the alarm circuit 20 reaches
a minimum value (indicating that the "secondary circuit" is formed
to resonance at the frequency of the signal being emitted by
oscillator 24). The condition of electrical resonance in the
"secondary circuit" of the sensor circuits 14,14 can be attributed
to the "distributed capacitance" characteristic exhibited by the
coil 16 in each sensor circuit. It will be understood that the
components of each alarm circuit 20,20 should be identical to those
used in the other alarm circuits 20,20 of the monitoring system and
that each alarm circuit 20,20 be operated at the same frequency.
Also, the components of each sensor circuit 14,14 should be
indentical to those used in the other sensor circuits 14,14 of the
monitoring system. When the "primary circuits" of the alarm
circuits 20,20 all are tuned to resonance at the frequency of the
signals generated by the respective oscillators 24,24 in the alarm
circuits 20,20 and the "secondary circuits" of the sensor circuits
14,14 tuned to resonance at the same frequency, the "primary
circuits" and the "secondary circuits" are considered to have
"matched resonance" and the passing of the inductor coils 16,16 of
the sensor circuits 14,14 in close proximity to the inductor coils
21,21 of the alarm circuits 20,20 will produce a noticeable change
in voltage across inductor coils 21,21 only when the "secondary
circuit" of a sensor circuit 14 is "open."
The frequency at which each alarm circuit 20,20 operates is
determined by the frequency of the signal generated by the
oscillator 24 in each alarm circuit. As indicated above, each alarm
circuit 20,20 is designed to operate at the same frequency.
Although the alarm circuits 20,20 can be designed to operate at
relatively low frequencies, normally it is desirable that the alarm
circuits 20,20 be designed to operate at a frequency above about
100 kilohertz.
During normal operation of the conveyor system 10, as the conveyor
belt 11 advances the inductor coils 16,16 of the sensor circuits
14,14 are advanced and sequentially pass in close proximity to the
inductor coils 21,21 of the alarm circuits 20,20 positioned at
spaced intervals along the path of travel of the conveyor belt 11.
As long as the circuit formed by the inductor coil 16 and wire loop
15 of a sensor circuit 14 is "closed," the passage of the sensor
circuit 14 in close proximity to an inductor coil 21 of an alarm
circuit 20 will have no noticeable effect on the alarm circuit 20
and the operation of the conveyor 10 will not be affected. However,
if a longitudinal rip develops in conveyor belt 11 and becomes of
such size as to cause the breaking of the loop of wire 15 of a
sensor circuit 14 (as is illustrated schematically in FIG. 1), the
circuit of such sensor circuit 14 will become "open" and the
inductor coil 16 of such sensor circuit 14 then will be in
resonance with its "distributed capacitance" and in "matched
resonance" with the "primary circuit" of an alarm circuit 20 during
the period that the inductor coil 16 of such sensor circuit 14
passes in close proximity to the inductor coil 21 of the alarm
circuit 14. As a consequence of the inductor coil 16 of such sensor
circuit 14 being in "matched resonance" with the "primary circuit"
of an alarm circuit 20 during the time interval that the conductor
16 of such sensor circuit 14 passes in close proxmity to the
inductor coil 21 of the alarm circuit 14, the resonance frequency
of the "primary circuit" of the alarm circuit 14 changes during the
period when the coil 16 is passing in close proximity to coil 21.
The aforementioned change of resonant frequency of the "primary
circuit" of the alarm circuit 20 from frequency f.sub.o to
frequency f.sub.o ' and the resulting drop in voltage amplitude
across the inductor coil 21 of the affected alarm circuit from
V.sub.o to V.sub.o ' is illustrated in FIG. 6. As is indicated by
the resonance curves in FIG. 6, when the electrical circuit of the
sensor circuit 14 is "closed," the voltage across inductor coil 21
of the alarm circuit 20 with the "primary circuit" of the alarm
circuit 20 in resonance at frequency f.sub.o is at maximum voltage
amplitude V.sub.o (shown by the solid line curve). However, a
longitudinal rip causes the breaking of the loop of wire 15 in a
sensor circuit 14 causing the electrical circuit of the sensor
circuit 14 to become "open," the resonant frequency of the "primary
circuit" of the alarm circuit 20 is changed to the new resonant
frequency f.sub.o ' during the period that the inductor coil 16 of
the sensor circuit having the broken loop of wire 15 passes in
close proximity to the inductor coil 21 of the alarm circuit (shown
by the dot and dash line curve). The frequency of the signal being
emitted by the oscillator 24 of the alarm circuit has not changed,
however, and remains at f.sub.o and, as illustrated in FIG. 6, the
voltage across the inductor coil 21 of the alarm circuit 20 drops
to voltage amplitude V.sub.o '. The drop in voltage across coil 21
(from V.sub.o to V.sub.o ') during the period that the inductor
coil 16 of the sensor circuit 14 with the broken wire loop 15
passes in close proximity to the inductor coil 21 of an alarm
circuit 20 is "noticed" by the triggering circuit 26 of the alarm
circuit and produces a warning signal or shuts down the conveyor
system or both.
The triggering circuit 26 may be any circuitry responsive to a
variation in voltage. One such triggering circuit is illustrated in
FIG. 4 and is composed of a field effect transistor (FET),
resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4, a silicon
controlled rectifier (SCR), a light emitting diode (LED), a buzzer,
switches S.sub.1 and S.sub.2, a relay switch (RS) and a DC power
source. As described above, when there are no broken sensor
circuits 14,14, the voltage across resistor 23 of the alarm circuit
is of small magnitude and insufficient to produce a signal of large
enough magnitude to trigger the SCR. However, when a longitudinal
rip occurs in the conveyor belt 11 and causes the breaking of a
wire loop 15 of a sensor circuit 14, the voltage across resistor 23
increases sufficiently (because of the drop in voltage across coil
21 of the alarm circuit) during the period that inductor coil 16 of
the open-circuited sensor circuit 14 passes in close proximity to
the inductor coil 21 of an alarm circuit 20 as the conveyor belt 11
advances to cause a signal of sufficient magnitude to be produced
to trigger the SCR. The buzzer and LED then produce audible aid
visual warning signals indicating the damaged condition of the
conveyor belt, which signals will continue until switches S.sub.1
and S.sub.2 are manually "opened." The triggering of the SCR also
energizes relay switch RS and breaks the circuit that includes the
motor for driving the conveyor system 10 thereby "shutting down"
the conveyor system 10 before further damage to the system
occurs.
As is evident from the foregoing description of an embodiment of
this invention, the "primary circuit" of each alarm circuit of the
monitoring system is designed to be tuned to electrical resonance
at the frequency at which the alarm circuits operate. Also, the
ratio of turns of wire in the inductor coil of each sensor circuit
of the monitoring system to the number of turns of wire in the
inductor coils of th alarm circuits is selected so that the
"secondary circuit" of the sensor circuits when the wire loop of
the sensor circuit is broken will be in electrical resonance at the
frequency at which the alarm circuits of the monitoring system
operate during the time that the inductor coil of such sensor
circuit (with a broken wire loop) passes in close proximity to an
inductor coil of an alarm circuit of the monitoring system as the
conveyor belt is advanced. The sensor circuits of the monitorng
system are devoid of condenser components, but, instead, depend
upon the "distributed capacitance" characteristic of the inductor
coil in each sensor circuit when circuit is open (as a result of
the breaking of the loop of wire in the circuit) to provide the
desired resonant condition in the circuit.
Although the preferred embodiment of the invention employs a series
of alarm circuits 20,20 spaced at intervals along the path of
advance of the conveyor belt, a single alarm circuit 20 may be
used, if desired. If only a single alarm circuit 20 is employed,
desirably it is located at a position along the path of advance of
the conveyor belt which monitors the conveyor belt soon after
material to be conveyed is charged onto the belt.
* * * * *